Hepatic Portal System

Certain medicines are always administered orally and cannot be administered via the intravenous or subcutaneous route. The only reason behind this is to activate the drug in the liver by the first pass mechanism. The system of our body which is responsible for executing such a process is the hepatic portal system.

A portal system is the part of the circulatory system in which blood is transported from the capillaries arising at one organ to the capillaries opening up into another organ through a vein. In simple language, we may say a portal system is one in which a vein begins and ends in capillaries. The hepatic portal system is associated with blood transport from the gastrointestinal tract to the liver before it is poured into the posterior vena cava.

What is the Hepatic Portal System?

The hepatic portal system is a system of veins that arise from the capillary bed of the stomach and end in the capillary bed of the liver. The hepatic portal circulation receives arterial blood from the aorta and venous blood from the gastrointestinal tract and the spleen. It is a complex system that allows the components absorbed into the blood through the digestive tract to reach the liver and get removed  before the blood reaches the heart through the posterior vena cava.

Hepatic Portal System Diagram

Hepatic Portal Vessels

The main hepatic portal vessel is the hepatic portal vein that arises from the union of gastric, superior mesenteric, inferior mesenteric, and splenic veins. The hepatic portal vein runs alongside the hepatic artery and the anterior bile duct in the gastro hepatoduodenal ligament and reaches the liver, which branches into capillaries that drain blood into the liver sinusoids. Interestingly, the hepatic portal vein cannot be considered a true vein because it does not carry blood to the heart.

Functions of the Hepatic Portal System

The major function of the hepatic portal system is to detoxify the blood before it reaches the heart. Apart from this, the blood that reaches the liver through the hepatic portal system contains nutrients and a surplus of glucose. These are either utilised to nourish the hepatocytes (liver cells) or stored in the liver for periods of crisis.

Difference Between Hepatic Vein and Portal Vein

There are two veins that circulate blood into or from the liver. They are the hepatic veins and the hepatic portal vein. The difference between them is summarised in the table below:

Significance of Hepatic Portal System

There are several toxic substances that are consumed with food and are absorbed in the digestive system. If not removed from the blood, these substances will reach the heart, and from there, they will get distributed to almost every part of the body. This may result in fatal damages. However, it is due to the hepatic portal system that these toxic substances get restricted in the liver or get converted into non-toxic substances before getting circulated.

The hepatic portal system is also responsible for providing three-fourths of the oxygen volume in the liver and only one-fourth is provided by the hepatic artery. Along with this, the hepatic portal system ensures the absorption of nutrients in the liver after the intestine and, in turn, directs these nutrients toward metabolism.

Disorders of the Hepatic Portal System

Portal hypertension is a condition characterised by unnaturally high blood pressure in the portal vein. There are two types of causes of such conditions: prehepatic and hepatic. Prehepatic causes include thrombosis of the hepatic portal and associated veins, hypersplenism, arteriovenous fistula, etc. Hepatic causes that occur in the liver include cirrhosis, chronic pancreatitis, hepatic fibrosis, etc. Out of these, liver cirrhosis is the most common cause.

Interesting Facts of Hepatic Portal System

• The fact that anything which is absorbed in the digestive system is carried to the liver by the hepatic portal system serves as the basis of the first-pass metabolism of a drug. 

• This character of the hepatic portal system mandates that certain drugs should be administered via a particular route. 

• Certain drugs like nitroglycerin must bypass the liver, so they are taken under the tongue or skin. 

• Some drugs like dextromethorphan need to reach the liver in order to get converted into the active compound. Thus, they are administered orally.

Important Questions

1. Suggest the name of the animals where the hepatic portal system is present.

Ans: Hepatic portal system comprises a group of veins around the intestine, stomach, spleen, and liver. This system is present in all mammals, amphibians, fishes, and reptiles. This system transports blood from the digestive tract  to the liver for detoxification and absorption.

2. Name the three portal systems.

Ans: The three portal system includes: 

• The hepatic portal system- present in the digestive tract.

• The hypophyseal portal system- present between the hypothalamus and anterior pituitary.

• The renal portal system- present in the kidney.

Key Features of Hepatic Portal System

• Hepatic portal system has its evolutionary significance that lies in the first pass mechanism, which ensures that anything absorbed by the digestive system does not reach the systemic circulation directly but passes through the liver. 

• Hepatic portal system carries blood from the alimentary canal to the liver.

• The main blood vessel of this system is not a true vein because it does not carry blood to the heart.

• This system plays a crucial role in the first-pass metabolism of drugs.

• The hepatic portal system brings in partially deoxygenated blood that meets the 60-70% oxygen need of the liver. Thus, the hepatic portal system is crucial to the circulatory system.